Internal Architecture of the Compiler
*************************************

This is meant to describe the C++ front-end for gcc in detail. Questions and comments to Benjamin Kosnik <bkoz@cygnus.com>.

Limitations of g++
==================

  * Limitations on input source code: 240 nesting levels with the parser stacksize (YYSTACKSIZE) set to 500 (the default), and requires around 16.4k swap space per nesting level.  The parser needs about 2.09 * number of nesting levels worth of stackspace.

  * I suspect there are other uses of pushdecl_class_level that do not call set_identifier_type_value in tandem with the call to pushdecl_class_level.  It would seem to be an omission.

  * Access checking is unimplemented for nested types.

  * volatile is not implemented in general.

Routines
========

This section describes some of the routines used in the C++ front-end.

build_vtable and prepare_fresh_vtable is used only within the cp-class.c file, and only in finish_struct and modify_vtable_entries.

build_vtable, prepare_fresh_vtable, and finish_struct are the only routines that set DECL_VPARENT.

finish_struct can steal the virtual function table from parents, this prohibits related_vslot from working.  When finish_struct steals, we know that

     get_binfo (DECL_FIELD_CONTEXT (CLASSTYPE_VFIELD (t)), t, 0)

will get the related binfo.

layout_basetypes does something with the VIRTUALS.

Supposedly (according to Tiemann) most of the breadth first searching done, like in get_base_distance and in get_binfo was not because of any design decision.  I have since found out the at least one part of the compiler needs the notion of depth first binfo searching, I am going to try and convert the whole thing, it should just work.  The term left-most refers to the depth first left-most node.  It uses MAIN_VARIANT == type as the condition to get left-most, because the things that have BINFO_OFFSETs of zero are shared and will have themselves as their own MAIN_VARIANTs.  The non-shared right ones, are copies of the left-most one, hence if it is its own MAIN_VARIANT, we know it IS a left-most one, if it is not, it is a non-left-most one.

get_base_distance's path and distance matters in its use in:

  * prepare_fresh_vtable (the code is probably wrong)

  * init_vfields Depends upon distance probably in a safe way, build_offset_ref might use partial paths to do further lookups, hack_identifier is probably not properly checking access.

  * get_first_matching_virtual probably should check for get_base_distance returning -2.

  * resolve_offset_ref should be called in a more deterministic manner.  Right now, it is called in some random contexts, like for arguments at build_method_call time, default_conversion time, convert_arguments time, build_unary_op time, build_c_cast time, build_modify_expr time, convert_for_assignment time, and convert_for_initialization time.

     But, there are still more contexts it needs to be called in, one was the ever simple:

          if (obj.*pmi != 7)
             ...

     Seems that the problems were due to the fact that TREE_TYPE of the OFFSET_REF was not a OFFSET_TYPE, but rather the type of the referent (like INTEGER_TYPE).  This problem was fixed by changing default_conversion to check TREE_CODE (x), instead of only checking TREE_CODE (TREE_TYPE (x)) to see if it was OFFSET_TYPE.

Implementation Specifics
========================

  * Explicit Initialization

     The global list current_member_init_list contains the list of mem-initializers specified in a constructor declaration.  For example:

          foo::foo() : a(1), b(2) {}

     will initialize a with 1 and b with 2. expand_member_init places each initialization (a with 1) on the global list.  Then, when the fndecl is being processed, emit_base_init runs down the list, initializing them.  It used to be the case that g++ first ran down current_member_init_list, then ran down the list of members initializing the ones that weren't explicitly initialized.  Things were rewritten to perform the initializations in order of declaration in the class.  So, for the above example, a and b will be initialized in the order that they were declared:

          class foo { public: int b; int a; foo (); };

     Thus, b will be initialized with 2 first, then a will be initialized with 1, regardless of how they're listed in the mem-initializer.

  * The Explicit Keyword

     The use of explicit on a constructor is used by grokdeclarator to set the field DECL_NONCONVERTING_P.  That value is used by build_method_call and build_user_type_conversion_1 to decide if a particular constructor should be used as a candidate for conversions.

Glossary
========

binfo
     The main data structure in the compiler used to represent the inheritance relationships between classes.  The data in the binfo can be accessed by the BINFO_ accessor macros.

vtable
virtual function table

     The virtual function table holds information used in virtual function dispatching.  In the compiler, they are usually referred to as vtables, or vtbls.  The first index is not used in the normal way, I believe it is probably used for the virtual destructor. There are two forms of virtual tables, one that has offsets in addition to pointers, and one using thunks. See Vtables.

vfield

     vfields can be thought of as the base information needed to build vtables.  For every vtable that exists for a class, there is a vfield. See also vtable and virtual function table pointer.  When a type is used as a base class to another type, the virtual function table for the derived class can be based upon the vtable for the base class, just extended to include the additional virtual methods declared in the derived class.  The virtual function table from a virtual base class is never reused in a derived class.  is_normal depends upon this.

virtual function table pointer

     These are FIELD_DECLs that are pointer types that point to vtables.  See also vtable and vfield.

Macros
======

This section describes some of the macros used on trees.  The list should be alphabetical.  Eventually all macros should be documented here.

BINFO_BASETYPES
     A vector of additional binfos for the types inherited by this basetype. The binfos are fully unshared (except for virtual bases, in which case the binfo structure is shared).

     If this basetype describes type D as inherited in C, and if the basetypes of D are E anf F, then this vector contains binfos for inheritance of E and F by C.

     Has values of:

     TREE_VECs

BINFO_INHERITANCE_CHAIN
     Temporarily used to represent specific inheritances.  It usually points to the binfo associated with the lesser derived type, but it can be reversed by reverse_path.  For example:

          	Z ZbY	least derived
          	|
          	Y YbX
          	|
          	X Xb	most derived

          TYPE_BINFO (X) == Xb
          BINFO_INHERITANCE_CHAIN (Xb) == YbX
          BINFO_INHERITANCE_CHAIN (Yb) == ZbY
          BINFO_INHERITANCE_CHAIN (Zb) == 0

     Not sure is the above is really true, get_base_distance has is point towards the most derived type, opposite from above.

     Set by build_vbase_path, recursive_bounded_basetype_p, get_base_distance, lookup_field, lookup_fnfields, and reverse_path.

     What things can this be used on:

     TREE_VECs that are binfos

BINFO_OFFSET
     The offset where this basetype appears in its containing type. BINFO_OFFSET slot holds the offset (in bytes) from the base of the complete object to the base of the part of the object that is allocated on behalf of this `type'.  This is always 0 except when there is multiple inheritance.

     Used on TREE_VEC_ELTs of the binfos BINFO_BASETYPES (...) for example.

BINFO_VIRTUALS
     A unique list of functions for the virtual function table.  See also TYPE_BINFO_VIRTUALS.

     What things can this be used on:

     TREE_VECs that are binfos

BINFO_VTABLE
     Used to find the VAR_DECL that is the virtual function table associated with this binfo.  See also TYPE_BINFO_VTABLE.  To get the virtual function table pointer, see CLASSTYPE_VFIELD.

     What things can this be used on:

     TREE_VECs that are binfos

     Has values of:

     VAR_DECLs that are virtual function tables

BLOCK_SUPERCONTEXT
     In the outermost scope of each function, it points to the FUNCTION_DECL node.  It aids in better DWARF support of inline functions.

CLASSTYPE_TAGS
     CLASSTYPE_TAGS is a linked (via TREE_CHAIN) list of member classes of a class. TREE_PURPOSE is the name, TREE_VALUE is the type (pushclass scans these and calls pushtag on them.)

     finish_struct scans these to produce TYPE_DECLs to add to the TYPE_FIELDS of the type.

     It is expected that name found in the TREE_PURPOSE slot is unique, resolve_scope_to_name is one such place that depends upon this uniqueness.

CLASSTYPE_METHOD_VEC
     The following is true after finish_struct has been called (on the class?) but not before.  Before finish_struct is called, things are different to some extent.  Contains a TREE_VEC of methods of the class. The TREE_VEC_LENGTH is the number of differently named methods plus one for the 0th entry.  The 0th entry is always allocated, and reserved for ctors and dtors.  If there are none, TREE_VEC_ELT(N,0) == NULL_TREE. Each entry of the TREE_VEC is a FUNCTION_DECL.  For each FUNCTION_DECL, there is a DECL_CHAIN slot.  If the FUNCTION_DECL is the last one with a given name, the DECL_CHAIN slot is NULL_TREE.  Otherwise it is the next method that has the same name (but a different signature).  It would seem that it is not true that because the DECL_CHAIN slot is used in this way, we cannot call pushdecl to put the method in the global scope (cause that would overwrite the TREE_CHAIN slot), because they use different _CHAINs.  finish_struct_methods setups up one version of the TREE_CHAIN slots on the FUNCTION_DECLs.

     friends are kept in TREE_LISTs, so that there's no need to use their TREE_CHAIN slot for anything.

     Has values of:

     TREE_VECs

CLASSTYPE_VFIELD
     Seems to be in the process of being renamed TYPE_VFIELD.  Use on types to get the main virtual function table pointer.  To get the virtual function table use BINFO_VTABLE (TYPE_BINFO ()).

     Has values of:

     FIELD_DECLs that are virtual function table pointers

     What things can this be used on:

     RECORD_TYPEs

DECL_CLASS_CONTEXT
     Identifies the context that the _DECL was found in.  For virtual function tables, it points to the type associated with the virtual function table.  See also DECL_CONTEXT, DECL_FIELD_CONTEXT and DECL_FCONTEXT.

     The difference between this and DECL_CONTEXT, is that for virtuals functions like:

          struct A
          {
            virtual int f ();
          };

          struct B : A
          {
            int f ();
          };

          DECL_CONTEXT (A::f) == A
          DECL_CLASS_CONTEXT (A::f) == A

          DECL_CONTEXT (B::f) == A
          DECL_CLASS_CONTEXT (B::f) == B

     Has values of:

     RECORD_TYPEs, or UNION_TYPEs

     What things can this be used on:

     TYPE_DECLs, _DECLs

DECL_CONTEXT
     Identifies the context that the _DECL was found in.  Can be used on virtual function tables to find the type associated with the virtual function table, but since they are FIELD_DECLs, DECL_FIELD_CONTEXT is a better access method.  Internally the same as DECL_FIELD_CONTEXT, so don't us both.  See also DECL_FIELD_CONTEXT, DECL_FCONTEXT and DECL_CLASS_CONTEXT.

     Has values of:

     RECORD_TYPEs

     What things can this be used on:

          VAR_DECLs that are virtual function tables
          _DECLs

DECL_FIELD_CONTEXT
     Identifies the context that the FIELD_DECL was found in.  Internally the same as DECL_CONTEXT, so don't us both.  See also DECL_CONTEXT, DECL_FCONTEXT and DECL_CLASS_CONTEXT.

     Has values of:

     RECORD_TYPEs

     What things can this be used on:

          FIELD_DECLs that are virtual function pointers
          FIELD_DECLs

DECL_NAME

     Has values of:

          0 for things that don't have names
          IDENTIFIER_NODEs for TYPE_DECLs

DECL_IGNORED_P
     A bit that can be set to inform the debug information output routines in the back-end that a certain _DECL node should be totally ignored.

     Used in cases where it is known that the debugging information will be output in another file, or where a sub-type is known not to be needed because the enclosing type is not needed.

     A compiler constructed virtual destructor in derived classes that do not define an explicit destructor that was defined explicit in a base class has this bit set as well.  Also used on __FUNCTION__ and __PRETTY_FUNCTION__ to mark they are ``compiler generated.''  c-decl and c-lex.c both want DECL_IGNORED_P set for ``internally generated vars,'' and ``user-invisible variable.''

     Functions built by the C++ front-end such as default destructors, virtual destructors and default constructors want to be marked that they are compiler generated, but unsure why.

     Currently, it is used in an absolute way in the C++ front-end, as an optimization, to tell the debug information output routines to not generate debugging information that will be output by another separately compiled file.

DECL_VIRTUAL_P
     A flag used on FIELD_DECLs and VAR_DECLs.  (Documentation in tree.h is wrong.)  Used in VAR_DECLs to indicate that the variable is a vtable. It is also used in FIELD_DECLs for vtable pointers.

     What things can this be used on:

     FIELD_DECLs and VAR_DECLs

DECL_VPARENT
     Used to point to the parent type of the vtable if there is one, else it is just the type associated with the vtable.  Because of the sharing of virtual function tables that goes on, this slot is not very useful, and is in fact, not used in the compiler at all.  It can be removed.

     What things can this be used on:

     VAR_DECLs that are virtual function tables

     Has values of:

     RECORD_TYPEs maybe UNION_TYPEs

DECL_FCONTEXT
     Used to find the first baseclass in which this FIELD_DECL is defined. See also DECL_CONTEXT, DECL_FIELD_CONTEXT and DECL_CLASS_CONTEXT.

     How it is used:

     Used when writing out debugging information about vfield and vbase decls.

     What things can this be used on:

     FIELD_DECLs that are virtual function pointers FIELD_DECLs

DECL_REFERENCE_SLOT
     Used to hold the initialize for the reference.

     What things can this be used on:

     PARM_DECLs and VAR_DECLs that have a reference type

DECL_VINDEX
     Used for FUNCTION_DECLs in two different ways.  Before the structure containing the FUNCTION_DECL is laid out, DECL_VINDEX may point to a FUNCTION_DECL in a base class which is the FUNCTION_DECL which this FUNCTION_DECL will replace as a virtual function.  When the class is laid out, this pointer is changed to an INTEGER_CST node which is suitable to find an index into the virtual function table.  See get_vtable_entry as to how one can find the right index into the virtual function table.  The first index 0, of a virtual function table it not used in the normal way, so the first real index is 1.

     DECL_VINDEX may be a TREE_LIST, that would seem to be a list of overridden FUNCTION_DECLs.  add_virtual_function has code to deal with this when it uses the variable base_fndecl_list, but it would seem that somehow, it is possible for the TREE_LIST to pursist until method_call, and it should not.

     What things can this be used on:

     FUNCTION_DECLs

DECL_SOURCE_FILE
     Identifies what source file a particular declaration was found in.

     Has values of:

     "<built-in>" on TYPE_DECLs to mean the typedef is built in

DECL_SOURCE_LINE
     Identifies what source line number in the source file the declaration was found at.

     Has values of:

          0 for an undefined label

          0 for TYPE_DECLs that are internally generated

          0 for FUNCTION_DECLs for functions generated by the compiler
          	(not yet, but should be)

          0 for ``magic'' arguments to functions, that the user has no
          	control over

TREE_USED

     Has values of:

     0 for unused labels

TREE_ADDRESSABLE
     A flag that is set for any type that has a constructor.

TREE_COMPLEXITY
     They seem a kludge way to track recursion, poping, and pushing.  They only appear in cp-decl.c and cp-decl2.c, so the are a good candidate for proper fixing, and removal.

TREE_HAS_CONSTRUCTOR
     A flag to indicate when a CALL_EXPR represents a call to a constructor. If set, we know that the type of the object, is the complete type of the object, and that the value returned is nonnull.  When used in this fashion, it is an optimization.  Can also be used on SAVE_EXPRs to indicate when they are of fixed type and nonnull.  Can also be used on INDIRECT_EXPRs on CALL_EXPRs that represent a call to a constructor.

TREE_PRIVATE
     Set for FIELD_DECLs by finish_struct.  But not uniformly set.

     The following routines do something with PRIVATE access: build_method_call, alter_access, finish_struct_methods, finish_struct, convert_to_aggr, CWriteLanguageDecl, CWriteLanguageType, CWriteUseObject, compute_access, lookup_field, dfs_pushdecl, GNU_xref_member, dbxout_type_fields, dbxout_type_method_1

TREE_PROTECTED
     The following routines do something with PROTECTED access: build_method_call, alter_access, finish_struct, convert_to_aggr, CWriteLanguageDecl, CWriteLanguageType, CWriteUseObject, compute_access, lookup_field, GNU_xref_member, dbxout_type_fields, dbxout_type_method_1

TYPE_BINFO
     Used to get the binfo for the type.

     Has values of:

     TREE_VECs that are binfos

     What things can this be used on:

     RECORD_TYPEs

TYPE_BINFO_BASETYPES
     See also BINFO_BASETYPES.

TYPE_BINFO_VIRTUALS
     A unique list of functions for the virtual function table.  See also BINFO_VIRTUALS.

     What things can this be used on:

     RECORD_TYPEs

TYPE_BINFO_VTABLE
     Points to the virtual function table associated with the given type. See also BINFO_VTABLE.

     What things can this be used on:

     RECORD_TYPEs

     Has values of:

     VAR_DECLs that are virtual function tables

TYPE_NAME
     Names the type.

     Has values of:

          0 for things that don't have names.
          should be IDENTIFIER_NODE for RECORD_TYPEs UNION_TYPEs and
                  ENUM_TYPEs.
          TYPE_DECL for RECORD_TYPEs, UNION_TYPEs and ENUM_TYPEs, but
                  shouldn't be.
          TYPE_DECL for typedefs, unsure why.

     What things can one use this on:

          TYPE_DECLs
          RECORD_TYPEs
          UNION_TYPEs
          ENUM_TYPEs

     History:

     It currently points to the TYPE_DECL for RECORD_TYPEs, UNION_TYPEs and ENUM_TYPEs, but it should be history soon.

TYPE_METHODS
     Synonym for CLASSTYPE_METHOD_VEC.  Chained together with TREE_CHAIN.  dbxout.c uses this to get at the methods of a class.

TYPE_DECL
     Used to represent typedefs, and used to represent bindings layers.

     Components:

     DECL_NAME is the name of the typedef.  For example, foo would be found in the DECL_NAME slot when typedef int foo; is seen.

     DECL_SOURCE_LINE identifies what source line number in the source file the declaration was found at.  A value of 0 indicates that this TYPE_DECL is just an internal binding layer marker, and does not correspond to a user supplied typedef.

     DECL_SOURCE_FILE

TYPE_FIELDS
     A linked list (via TREE_CHAIN) of member types of a class.  The list can contain TYPE_DECLs, but there can also be other things in the list apparently.  See also CLASSTYPE_TAGS.

TYPE_VIRTUAL_P
     A flag used on a FIELD_DECL or a VAR_DECL, indicates it is a virtual function table or a pointer to one.  When used on a FUNCTION_DECL, indicates that it is a virtual function.  When used on an IDENTIFIER_NODE, indicates that a function with this same name exists and has been declared virtual.

     When used on types, it indicates that the type has virtual functions, or is derived from one that does.

     Not sure if the above about virtual function tables is still true.  See also info on DECL_VIRTUAL_P.

     What things can this be used on:

     FIELD_DECLs, VAR_DECLs, FUNCTION_DECLs, IDENTIFIER_NODEs

VF_BASETYPE_VALUE
     Get the associated type from the binfo that caused the given vfield to exist.  This is the least derived class (the most parent class) that needed a virtual function table.  It is probably the case that all uses of this field are misguided, but they need to be examined on a case-by-case basis.  See history for more information on why the previous statement was made.

     Set at finish_base_struct time.

     What things can this be used on:

     TREE_LISTs that are vfields

     History:

     This field was used to determine if a virtual function table's slot should be filled in with a certain virtual function, by checking to see if the type returned by VF_BASETYPE_VALUE was a parent of the context in which the old virtual function existed. This incorrectly assumes that a given type _could_ not appear as a parent twice in a given inheritance lattice.  For single inheritance, this would in fact work, because a type could not possibly appear more than once in an inheritance lattice, but with multiple inheritance, a type can appear more than once.

VF_BINFO_VALUE
     Identifies the binfo that caused this vfield to exist.  If this vfield is from the first direct base class that has a virtual function table, then VF_BINFO_VALUE is NULL_TREE, otherwise it will be the binfo of the direct base where the vfield came from.  Can use TREE_VIA_VIRTUAL on result to find out if it is a virtual base class.  Related to the binfo found by

          get_binfo (VF_BASETYPE_VALUE (vfield), t, 0)

     where t is the type that has the given vfield.

          get_binfo (VF_BASETYPE_VALUE (vfield), t, 0)

     will return the binfo for the given vfield.

     May or may not be set at modify_vtable_entries time.  Set at finish_base_struct time.

     What things can this be used on:

     TREE_LISTs that are vfields

VF_DERIVED_VALUE
     Identifies the type of the most derived class of the vfield, excluding the class this vfield is for.

     Set at finish_base_struct time.

     What things can this be used on:

     TREE_LISTs that are vfields

VF_NORMAL_VALUE
     Identifies the type of the most derived class of the vfield, including the class this vfield is for.

     Set at finish_base_struct time.

     What things can this be used on:

     TREE_LISTs that are vfields

WRITABLE_VTABLES
     This is a option that can be defined when building the compiler, that will cause the compiler to output vtables into the data segment so that the vtables maybe written.  This is undefined by default, because normally the vtables should be unwritable.  People that implement object I/O facilities may, or people that want to change the dynamic type of objects may want to have the vtables writable.  Another way of achieving this would be to make a copy of the vtable into writable memory, but the drawback there is that that method only changes the type for one object.

Typical Behavior
================

Whenever seemingly normal code fails with errors like syntax error at `\{', it's highly likely that grokdeclarator is returning a NULL_TREE for whatever reason.

Coding Conventions
==================

It should never be that case that trees are modified in-place by the back-end, unless it is guaranteed that the semantics are the same no matter how shared the tree structure is.  fold-const.c still has some cases where this is not true, but rms hypothesizes that this will never be a problem.

Templates
=========

A template is represented by a TEMPLATE_DECL.  The specific fields used are:

DECL_TEMPLATE_RESULT
     The generic decl on which instantiations are based.  This looks just like any other decl.

DECL_TEMPLATE_PARMS
     The parameters to this template.

The generic decl is parsed as much like any other decl as possible, given the parameterization.  The template decl is not built up until the generic decl has been completed.  For template classes, a template decl is generated for each member function and static data member, as well.

Template members of template classes are represented by a TEMPLATE_DECL for the class' parameters around another TEMPLATE_DECL for the member's parameters.

All declarations that are instantiations or specializations of templates refer to their template and parameters through DECL_TEMPLATE_INFO.

How should I handle parsing member functions with the proper param decls?  Set them up again or try to use the same ones?  Currently we do the former.  We can probably do this without any extra machinery in store_pending_inline, by deducing the parameters from the decl in do_pending_inlines.  PRE_PARSED_TEMPLATE_DECL?

If a base is a parm, we can't check anything about it.  If a base is not a parm, we need to check it for name binding.  Do finish_base_struct if no bases are parameterized (only if none, including indirect, are parms).  Nah, don't bother trying to do any of this until instantiation -- we only need to do name binding in advance.

Always set up method vec and fields, inc. synthesized methods.  Really? We can't know the types of the copy folks, or whether we need a destructor, or can have a default ctor, until we know our bases and fields.  Otherwise, we can assume and fix ourselves later.  Hopefully.

Access Control
==============

The function compute_access returns one of three values:

access_public
     means that the field can be accessed by the current lexical scope.

access_protected
     means that the field cannot be accessed by the current lexical scope because it is protected.

access_private
     means that the field cannot be accessed by the current lexical scope because it is private.

DECL_ACCESS is used for access declarations; alter_access creates a list of types and accesses for a given decl.

Formerly, DECL_{PUBLIC,PROTECTED,PRIVATE} corresponded to the return codes of compute_access and were used as a cache for compute_access. Now they are not used at all.

TREE_PROTECTED and TREE_PRIVATE are used to record the access levels granted by the containing class.  BEWARE: TREE_PUBLIC means something completely unrelated to access control!

Error Reporting
===============

The C++ front-end uses a call-back mechanism to allow functions to print out reasonable strings for types and functions without putting extra logic in the functions where errors are found.  The interface is through the cp_error function (or cp_warning, etc.).  The syntax is exactly like that of error, except that a few more conversions are supported:

  * %C indicates a value of `enum tree_code'.

  * %D indicates a *_DECL node.

  * %E indicates a *_EXPR node.

  * %L indicates a value of `enum languages'.

  * %P indicates the name of a parameter (i.e. "this", "1", "2", ...)

  * %T indicates a *_TYPE node.

  * %O indicates the name of an operator (MODIFY_EXPR -> "operator =").

There is some overlap between these; for instance, any of the node options can be used for printing an identifier (though only %D tries to decipher function names).

For a more verbose message (class foo as opposed to just foo, including the return type for functions), use %#c. To have the line number on the error message indicate the line of the DECL, use cp_error_at and its ilk; to indicate which argument you want, use %+D, or it will default to the first.

Parser
======

Some comments on the parser:

The after_type_declarator / notype_declarator hack is necessary in order to allow redeclarations of TYPENAMEs, for instance

     typedef int foo;
     class A {
       char *foo;
     };

In the above, the first foo is parsed as a notype_declarator, and the second as a after_type_declarator.

Ambiguities:

There are currently four reduce/reduce ambiguities in the parser.  They are:

1) Between template_parm and named_class_head_sans_basetype, for the tokens aggr identifier.  This situation occurs in code looking like

     template <class T> class A { };

It is ambiguous whether class T should be parsed as the declaration of a template type parameter named T or an unnamed constant parameter of type class T.  Section 14.6, paragraph 3 of the January '94 working paper states that the first interpretation is the correct one.  This ambiguity results in two reduce/reduce conflicts.

2) Between primary and type_id for code like int() in places where both can be accepted, such as the argument to sizeof.  Section 8.1 of the pre-San Diego working paper specifies that these ambiguous constructs will be interpreted as typenames. This ambiguity results in six reduce/reduce conflicts between absdcl and functional_cast.

3) Between functional_cast and complex_direct_notype_declarator, for various token strings. This situation occurs in code looking like

     int (*a);

This code is ambiguous; it could be a declaration of the variable a as a pointer to int, or it could be a functional cast of *a to int.  Section 6.8 specifies that the former interpretation is correct.  This ambiguity results in 7 reduce/reduce conflicts.  Another aspect of this ambiguity is code like 'int (x[2]);', which is resolved at the '[' and accounts for 6 reduce/reduce conflicts between direct_notype_declarator and primary/overqualified_id.  Finally, there are 4 r/r conflicts between expr_or_declarator and primary over code like 'int (a);', which could probably be resolved but would also probably be more trouble than it's worth.  In all, this situation accounts for 17 conflicts.  Ack!

The second case above is responsible for the failure to parse 'LinppFile ppfile (String (argv[1]), &outs, argc, argv);' (from Rogue Wave Math.h++) as an object declaration, and must be fixed so that it does not resolve until later.

4) Indirectly between after_type_declarator and parm, for type names.  This occurs in (as one example) code like

     typedef int foo, bar;
     class A {
       foo (bar);
     };

What is bar inside the class definition?  We currently interpret it as a parm, as does Cfront, but IBM xlC interprets it as an after_type_declarator.  I believe that xlC is correct, in light of 7.1p2, which says "The longest sequence of decl-specifiers that could possibly be a type name is taken as the decl-specifier-seq of a declaration."  However, it seems clear that this rule must be violated in the case of constructors.  This ambiguity accounts for 8 conflicts.

Unlike the others, this ambiguity is not recognized by the Working Paper.

Exception Handling
==================

Note, exception handling in g++ is still under development.

This section describes the mapping of C++ exceptions in the C++ front-end, into the back-end exception handling framework.

The basic mechanism of exception handling in the back-end is unwind-protect a la elisp.  This is a general, robust, and language independent representation for exceptions.

The C++ front-end exceptions are mapping into the unwind-protect semantics by the C++ front-end.  The mapping is describe below.

When -frtti is used, rtti is used to do exception object type checking, when it isn't used, the encoded name for the type of the object being thrown is used instead.  All code that originates exceptions, even code that throws exceptions as a side effect, like dynamic casting, and all code that catches exceptions must be compiled with either -frtti, or -fno-rtti.  It is not possible to mix rtti base exception handling objects with code that doesn't use rtti.  The exceptions to this, are code that doesn't catch or throw exceptions, catch (...), and code that just rethrows an exception.

Currently we use the normal mangling used in building functions names (int's are "i", const char * is PCc) to build the non-rtti base type descriptors for exception handling.  These descriptors are just plain NULL terminated strings, and internally they are passed around as char *.

In C++, all cleanups should be protected by exception regions.  The region starts just after the reason why the cleanup is created has ended.  For example, with an automatic variable, that has a constructor, it would be right after the constructor is run.  The region ends just before the finalization is expanded.  Since the backend may expand the cleanup multiple times along different paths, once for normal end of the region, once for non-local gotos, once for returns, etc, the backend must take special care to protect the finalization expansion, if the expansion is for any other reason than normal region end, and it is `inline' (it is inside the exception region).  The backend can either choose to move them out of line, or it can created an exception region over the finalization to protect it, and in the handler associated with it, it would not run the finalization as it otherwise would have, but rather just rethrow to the outer handler, careful to skip the normal handler for the original region.

In Ada, they will use the more runtime intensive approach of having fewer regions, but at the cost of additional work at run time, to keep a list of things that need cleanups.  When a variable has finished construction, they add the cleanup to the list, when the come to the end of the lifetime of the variable, the run the list down.  If the take a hit before the section finishes normally, they examine the list for actions to perform.  I hope they add this logic into the back-end, as it would be nice to get that alternative approach in C++.

On an rs6000, xlC stores exception objects on that stack, under the try block.  When is unwinds down into a handler, the frame pointer is adjusted back to the normal value for the frame in which the handler resides, and the stack pointer is left unchanged from the time at which the object was thrown.  This is so that there is always someplace for the exception object, and nothing can overwrite it, once we start throwing.  The only bad part, is that the stack remains large.

The below points out some things that work in g++'s exception handling.

All completely constructed temps and local variables are cleaned up in all unwinded scopes.  Completely constructed parts of partially constructed objects are cleaned up.  This includes partially built arrays.  Exception specifications are now handled.  Thrown objects are now cleaned up all the time.  We can now tell if we have an active exception being thrown or not (__eh_type != 0).  We use this to call terminate if someone does a throw; without there being an active exception object.  uncaught_exception () works.  Exception handling should work right if you optimize.  Exception handling should work with -fpic or -fPIC.

The below points out some flaws in g++'s exception handling, as it now stands.

Only exact type matching or reference matching of throw types works when -fno-rtti is used.  Only works on a SPARC (like Suns) (both -mflat and -mno-flat models work), SPARClite, Hitachi SH, i386, arm, rs6000, PowerPC, Alpha, mips, VAX, m68k and z8k machines.  SPARC v9 may not work.  HPPA is mostly done, but throwing between a shared library and user code doesn't yet work.  Some targets have support for data-driven unwinding.  Partial support is in for all other machines, but a stack unwinder called __unwind_function has to be written, and added to libgcc2 for them.  The new EH code doesn't rely upon the __unwind_function for C++ code, instead it creates per function unwinders right inside the function, unfortunately, on many platforms the definition of RETURN_ADDR_RTX in the tm.h file for the machine port is wrong.  See below for details on __unwind_function.  RTL_EXPRs for EH cond variables for && and || exprs should probably be wrapped in UNSAVE_EXPRs, and RTL_EXPRs tweaked so that they can be unsaved.

We only do pointer conversions on exception matching a la 15.3 p2 case 3: `A handler with type T, const T, T&, or const T& is a match for a throw-expression with an object of type E if [3]T is a pointer type and E is a pointer type that can be converted to T by a standard pointer conversion (_conv.ptr_) not involving conversions to pointers to private or protected base classes.' when -frtti is given.

We don't call delete on new expressions that die because the ctor threw an exception.  See except/18 for a test case.

15.2 para 13: The exception being handled should be rethrown if control reaches the end of a handler of the function-try-block of a constructor or destructor, right now, it is not.

15.2 para 12: If a return statement appears in a handler of function-try-block of a constructor, the program is ill-formed, but this isn't diagnosed.

15.2 para 11: If the handlers of a function-try-block contain a jump into the body of a constructor or destructor, the program is ill-formed, but this isn't diagnosed.

15.2 para 9: Check that the fully constructed base classes and members of an object are destroyed before entering the handler of a function-try-block of a constructor or destructor for that object.

build_exception_variant should sort the incoming list, so that it implements set compares, not exact list equality.  Type smashing should smash exception specifications using set union.

Thrown objects are usually allocated on the heap, in the usual way.  If one runs out of heap space, throwing an object will probably never work. This could be relaxed some by passing an __in_chrg parameter to track who has control over the exception object.  Thrown objects are not allocated on the heap when they are pointer to object types.  We should extend it so that all small (<4*sizeof(void*)) objects are stored directly, instead of allocated on the heap.

When the backend returns a value, it can create new exception regions that need protecting.  The new region should rethrow the object in context of the last associated cleanup that ran to completion.

The structure of the code that is generated for C++ exception handling code is shown below:

     Ln:					throw value;
             copy value onto heap
             jump throw (Ln, id, address of copy of value on heap)

                                             try {
     +Lstart:	the start of the main EH region
     |...						...
     +Lend:		the end of the main EH region
                                             } catch (T o) {
     						...1
                                             }
     Lresume:
             nop	used to make sure there is something before
                     the next region ends, if there is one
     ...                                     ...

             jump Ldone
     [
     Lmainhandler:    handler for the region Lstart-Lend
     	cleanup
     ] zero or more, depending upon automatic vars with dtors
     +Lpartial:
     |        jump Lover
     +Lhere:
             rethrow (Lhere, same id, same obj);
     Lterm:		handler for the region Lpartial-Lhere
             call terminate
     Lover:
     [
      [
             call throw_type_match
             if (eq) {
      ] these lines disappear when there is no catch condition
     +Lsregion2:
     |	...1
     |	jump Lresume
     |Lhandler:	handler for the region Lsregion2-Leregion2
     |	rethrow (Lresume, same id, same obj);
     +Leregion2
             }
     ] there are zero or more of these sections, depending upon how many
       catch clauses there are
     ----------------------------- expand_end_all_catch --------------------------
                     here we have fallen off the end of all catch
                     clauses, so we rethrow to outer
             rethrow (Lresume, same id, same obj);
     ----------------------------- expand_end_all_catch --------------------------
     [
     L1:     maybe throw routine
     ] depending upon if we have expanded it or not
     Ldone:
             ret

     start_all_catch emits labels: Lresume,

The __unwind_function takes a pointer to the throw handler, and is expected to pop the stack frame that was built to call it, as well as the frame underneath and then jump to the throw handler.  It must restore all registers to their proper values as well as all other machine state as determined by the context in which we are unwinding into.  The way I normally start is to compile:

void *g; foo(void* a) { g = a; }

with -S, and change the thing that alters the PC (return, or ret usually) to not alter the PC, making sure to leave all other semantics (like adjusting the stack pointer, or frame pointers) in.  After that, replicate the prologue once more at the end, again, changing the PC altering instructions, and finally, at the very end, jump to `g'.

It takes about a week to write this routine, if someone wants to volunteer to write this routine for any architecture, exception support for that architecture will be added to g++.  Please send in those code donations.  One other thing that needs to be done, is to double check that __builtin_return_address (0) works.

Specific Targets
----------------

For the alpha, the __unwind_function will be something resembling:

     void
     __unwind_function(void *ptr)
     {
       /* First frame */
       asm ("ldq $15, 8($30)"); /* get the saved frame ptr; 15 is fp, 30 is sp */
       asm ("bis $15, $15, $30"); /* reload sp with the fp we found */

       /* Second frame */
       asm ("ldq $15, 8($30)"); /* fp */
       asm ("bis $15, $15, $30"); /* reload sp with the fp we found */

       /* Return */
       asm ("ret $31, ($16), 1"); /* return to PTR, stored in a0 */
     }

However, there are a few problems preventing it from working.  First of all, the gcc-internal function __builtin_return_address needs to work given an argument of 0 for the alpha.  As it stands as of August 30th, 1995, the code for BUILT_IN_RETURN_ADDRESS in expr.c will definitely not work on the alpha.  Instead, we need to define the macros DYNAMIC_CHAIN_ADDRESS (maybe), RETURN_ADDR_IN_PREVIOUS_FRAME, and definitely need a new definition for RETURN_ADDR_RTX.

In addition (and more importantly), we need a way to reliably find the frame pointer on the alpha.  The use of the value 8 above to restore the frame pointer (register 15) is incorrect.  On many systems, the frame pointer is consistently offset to a specific point on the stack.  On the alpha, however, the frame pointer is pushed last.  First the return address is stored, then any other registers are saved (e.g., s0), and finally the frame pointer is put in place.  So fp could have an offset of 8, but if the calling function saved any registers at all, they add to the offset.

The only places the frame size is noted are with the .frame directive, for use by the debugger and the OSF exception handling model (useless to us), and in the initial computation of the new value for sp, the stack pointer.  For example, the function may start with:

     lda $30,-32($30)
     .frame $15,32,$26,0

The 32 above is exactly the value we need.  With this, we can be sure that the frame pointer is stored 8 bytes less---in this case, at 24(sp)). The drawback is that there is no way that I (Brendan) have found to let us discover the size of a previous frame inside the definition of __unwind_function.

So to accomplish exception handling support on the alpha, we need two things: first, a way to figure out where the frame pointer was stored, and second, a functional __builtin_return_address implementation for except.c to be able to use it.

Or just support DWARF 2 unwind info.

New Backend Exception Support
-----------------------------

This subsection discusses various aspects of the design of the data-driven model being implemented for the exception handling backend.

The goal is to generate enough data during the compilation of user code, such that we can dynamically unwind through functions at run time with a single routine (__throw) that lives in libgcc.a, built by the compiler, and dispatch into associated exception handlers.

This information is generated by the DWARF 2 debugging backend, and includes all of the information __throw needs to unwind an arbitrary frame.  It specifies where all of the saved registers and the return address can be found at any point in the function.

Major disadvantages when enabling exceptions are:

  * Code that uses caller saved registers, can't, when flow can be transferred into that code from an exception handler.  In high performance code this should not usually be true, so the effects should be minimal.

Backend Exception Support
-------------------------

The backend must be extended to fully support exceptions.  Right now there are a few hooks into the alpha exception handling backend that resides in the C++ frontend from that backend that allows exception handling to work in g++.  An exception region is a segment of generated code that has a handler associated with it.  The exception regions are denoted in the generated code as address ranges denoted by a starting PC value and an ending PC value of the region.  Some of the limitations with this scheme are:

  * The backend replicates insns for such things as loop unrolling and function inlining.  Right now, there are no hooks into the frontend's exception handling backend to handle the replication of insns.  When replication happens, a new exception region descriptor needs to be generated for the new region.

  * The backend expects to be able to rearrange code, for things like jump optimization.  Any rearranging of the code needs have exception region descriptors updated appropriately.

  * The backend can eliminate dead code.  Any associated exception region descriptor that refers to fully contained code that has been eliminated should also be removed, although not doing this is harmless in terms of semantics.

The above is not meant to be exhaustive, but does include all things I have thought of so far.  I am sure other limitations exist.

Below are some notes on the migration of the exception handling code backend from the C++ frontend to the backend.

NOTEs are to be used to denote the start of an exception region, and the end of the region.  I presume that the interface used to generate these notes in the backend would be two functions, start_exception_region and end_exception_region (or something like that).  The frontends are required to call them in pairs.  When marking the end of a region, an argument can be passed to indicate the handler for the marked region. This can be passed in many ways, currently a tree is used.  Another possibility would be insns for the handler, or a label that denotes a handler.  I have a feeling insns might be the best way to pass it. Semantics are, if an exception is thrown inside the region, control is transferred unconditionally to the handler.  If control passes through the handler, then the backend is to rethrow the exception, in the context of the end of the original region.  The handler is protected by the conventional mechanisms; it is the frontend's responsibility to protect the handler, if special semantics are required.

This is a very low level view, and it would be nice is the backend supported a somewhat higher level view in addition to this view.  This higher level could include source line number, name of the source file, name of the language that threw the exception and possibly the name of the exception.  Kenner may want to rope you into doing more than just the basics required by C++.  You will have to resolve this.  He may want you to do support for non-local gotos, first scan for exception handler, if none is found, allow the debugger to be entered, without any cleanups being done.  To do this, the backend would have to know the difference between a cleanup-rethrower, and a real handler, if would also have to have a way to know if a handler `matches' a thrown exception, and this is frontend specific.

The stack unwinder is one of the hardest parts to do.  It is highly machine dependent.  The form that kenner seems to like was a couple of macros, that would do the machine dependent grunt work.  One preexisting function that might be of some use is __builtin_return_address ().  One macro he seemed to want was __builtin_return_address, and the other would do the hard work of fixing up the registers, adjusting the stack pointer, frame pointer, arg pointer and so on.

Free Store
==========

operator new [] adds a magic cookie to the beginning of arrays for which the number of elements will be needed by operator delete [].  These are arrays of objects with destructors and arrays of objects that define operator delete [] with the optional size_t argument. This cookie can be examined from a program as follows:

     typedef unsigned long size_t;
     extern "C" int printf (const char *, ...);

     size_t nelts (void *p)
     {
       struct cookie {
         size_t nelts __attribute__ ((aligned (sizeof (double))));
       };

       cookie *cp = (cookie *)p;
       --cp;

       return cp->nelts;
     }

     struct A {
       ~A() { }
     };

     main()
     {
       A *ap = new A[3];
       printf ("%ld\n", nelts (ap));
     }

Linkage
=======

The linkage code in g++ is horribly twisted in order to meet two design goals:

1) Avoid unnecessary emission of inlines and vtables.

2) Support pedantic assemblers like the one in AIX.

To meet the first goal, we defer emission of inlines and vtables until the end of the translation unit, where we can decide whether or not they are needed, and how to emit them if they are.

Function name mangling for C++ and Java
=======================================

Both C++ and Jave provide overloaded function and methods, which are methods with the same types but different parameter lists. Selecting the correct version is done at compile time. Though the overloaded functions have the same name in the source code, they need to be translated into different assembler-level names, since typical assemblers and linkers cannot handle overloading. This process of encoding the parameter types with the method name into a unique name is called name mangling.  The inverse process is called demangling.

It is convenient that C++ and Java use compatible mangling schemes, since the makes life easier for tools such as gdb, and it eases integration between C++ and Java.

Note there is also a standard "Jave Native Interface" (JNI) which implements a different calling convention, and uses a different mangling scheme.  The JNI is a rather abstract ABI so Java can call methods written in C or C++; we are concerned here about a lower-level interface primarily intended for methods written in Java, but that can also be used for C++ (and less easily C).

Note that on systems that follow BSD tradition, a C identifier var would get "mangled" into the assembler name _var.  On such systems, all other mangled names are also prefixed by a _ which is not shown in the following examples.

Method name mangling
--------------------

C++ mangles a method by emitting the function name, followed by __, followed by encodings of any method qualifiers (such as const), followed by the mangling of the method's class, followed by the mangling of the parameters, in order.

For example Foo::bar(int, long) const is mangled as bar__C3Fooil.

For a constructor, the method name is left out. That is Foo::Foo(int, long) const  is mangled as __C3Fooil.

GNU Java does the same.

Primitive types
---------------

The C++ types int, long, short, char, and long long are mangled as i, l, s, c, and x, respectively. The corresponding unsigned types have U prefixed to the mangling.  The type signed char is mangled Sc.

The C++ and Java floating-point types float and double are mangled as f and d respectively.

The C++ bool type and the Java boolean type are mangled as b.

The C++ wchar_t and the Java char types are mangled as w.

The Java integral types byte, short, int and long are mangled as c, s, i, and x, respectively.

C++ code that has included javatypes.h will mangle the typedefs  jbyte, jshort, jint and jlong as respectively c, s, i, and x.  (This has not been implemented yet.)

Mangling of simple names
------------------------

A simple class, package, template, or namespace name is encoded as the number of characters in the name, followed by the actual characters.  Thus the class Foo is encoded as 3Foo.

If any of the characters in the name are not alphanumeric (i.e not one of the standard ASCII letters, digits, or '_'), or the initial character is a digit, then the name is mangled as a sequence of encoded Unicode letters. A Unicode encoding starts with a U to indicate that Unicode escapes are used, followed by the number of bytes used by the Unicode encoding, followed by the bytes representing the encoding.  ASSCI letters and non-initial digits are encoded without change.  However, all other characters (including underscore and initial digits) are translated into a sequence starting with an underscore, followed by the big-endian 4-hex-digit lower-case encoding of the character.

If a method name contains Unicode-escaped characters, the entire mangled method name is followed by a U.

For example, the method X\u0319::M\u002B(int) is encoded as M_002b__U6X_0319iU.

Pointer and reference types
---------------------------

A C++ pointer type is mangled as P followed by the mangling of the type pointed to.

A C++ reference type as mangled as R followed by the mangling of the type referenced.

A Java object reference type is equivalent to a C++ pointer parameter, so we mangle such an parameter type as P followed by the mangling of the class name.

Squangled type compression
--------------------------

Squangling (enabled with the -fsquangle option), utilizes the B code to indicate reuse of a previously seen type within an indentifier. Types are recognized in a left to right manner and given increasing values, which are appended to the code in the standard manner. Ie, multiple digit numbers are delimited by _ characters. A type is considered to be any non primitive type, regardless of whether its a parameter, template parameter, or entire template. Certain codes are considered modifiers of a type, and are not included as part of the type. These are the C, V, P, A, R, U and u codes, denoting constant, volatile, pointer, array, reference, unsigned, and restrict. These codes may precede a B type in order to make the required modifications to the type.

For example:

     template <class T> class class1 { };

     template <class T> class class2 { };

     class class3 { };

     int f(class2<class1<class3> > a ,int b, const class1<class3>&c, class3 *d) { }

         B0 -> class2<class1<class3>
         B1 -> class1<class3>
         B2 -> class3

Produces the mangled name f__FGt6class21Zt6class11Z6class3iRCB1PB2. The int parameter is a basic type, and does not receive a B encoding...

Qualified names
---------------

Both C++ and Java allow a class to be lexically nested inside another class.  C++ also supports namespaces (not yet implemented by G++). Java also supports packages.

These are all mangled the same way:  First the letter Q indicates that we are emitting a qualified name. That is followed by the number of parts in the qualified name. If that number is 9 or less, it is emitted with no delimiters. Otherwise, an underscore is written before and after the count. Then follows each part of the qualified name, as described above.

For example Foo::\u0319::Bar is encoded as Q33FooU5_03193Bar.

Squangling utilizes the the letter K to indicate a remembered portion of a qualified name. As qualified names are processed for an identifier, the names are numbered and remembered in a manner similar to the B type compression code. Names are recognized left to right, and given increasing values, which are appended to the code in the standard manner. ie, multiple digit numbers are delimited by _ characters.

For example

     class Andrew
     {
       class WasHere
       {
           class AndHereToo
           {
           };
       };
     };

     f(Andrew&r1, Andrew::WasHere& r2, Andrew::WasHere::AndHereToo& r3) { }

        K0 ->  Andrew
        K1 ->  Andrew::WasHere
        K2 ->  Andrew::WasHere::AndHereToo

Function f() would be mangled as : f__FR6AndrewRQ2K07WasHereRQ2K110AndHereToo

There are some occasions when either a B or K code could be chosen, preference is always given to the B code. Ie, the example in the section on B mangling could have used a K code instead of B2.

Templates
---------

A class template instantiation is encoded as the letter t, followed by the encoding of the template name, followed the number of template parameters, followed by encoding of the template parameters.  If a template parameter is a type, it is written as a Z followed by the encoding of the type.

A function template specialization (either an instantiation or an explicit specialization) is encoded by an H followed by the encoding of the template parameters, as described above, followed by an _, the encoding of the argument types to the template function (not the specialization), another _, and the return type.  (Like the argument types, the return type is the return type of the function template, not the specialization.)  Template parameters in the argument and return types are encoded by an X for type parameters, or a Y for constant parameters, an index indicating their position in the template parameter list declaration, and their template depth.

Arrays
------

C++ array types are mangled by emitting A, followed by the length of the array, followed by an _, followed by the mangling of the element type.  Of course, normally array parameter types decay into a pointer types, so you don't see this.

Java arrays are objects.  A Java type T[] is mangled as if it were the C++ type JArray<T>. For example java.lang.String[] is encoded as Pt6JArray1ZPQ34java4lang6String.

Static fields
-------------

Both C++ and Java classes can have static fields. These are allocated statically, and are shared among all instances.

The mangling starts with a prefix (_ in most systems), which is followed by the mangling of the class name, followed by the "joiner" and finally the field name. The joiner (see JOINER in cp-tree.h) is a special separator character.  For historical reasons (and idiosyncracies of assembler syntax) it can $ or . (or even _ on a few systems).  If the joiner is _ then the prefix is __static_ instead of just _.

For example Foo::Bar::var (or Foo.Bar.var in Java syntax) would be encoded as _Q23Foo3Bar$var or _Q23Foo3Bar.var (or rarely __static_Q23Foo3Bar_var).

If the name of a static variable needs Unicode escapes, the Unicode indicator U comes before the "joiner". This \u1234Foo::var\u3445 becomes _U8_1234FooU.var_3445.

Table of demangling code characters
-----------------------------------

The following special characters are used in mangling:

A
     Indicates a C++ array type.

b
     Encodes the C++ bool type, and the Java boolean type.

B
     Used for squangling. Similar in concept to the 'T' non-squangled code.

c
     Encodes the C++ char type, and the Java byte type.

C
     A modifier to indicate a const type. Also used to indicate a const member function (in which cases it precedes the encoding of the method's class).

d
     Encodes the C++ and Java double types.

e
     Indicates extra unknown arguments ....

E
     Indicates the opening parenthesis of an expression.

f
     Encodes the C++ and Java float types.

F
     Used to indicate a function type.

H
     Used to indicate a template function.

i
     Encodes the C++ and Java int types.

I
     Encodes typedef names of the form intN_t, where N is a positive decimal number.  The I is followed by either two hexidecimal digits, which encode the value of N, or by an arbitrary number of hexidecimal digits between underscores.  For example, I40 encodes the type int64_t, and I_200_ encodes the type int512_t.

J
     Indicates a complex type.

K
     Used by squangling to compress qualified names.

l
     Encodes the C++ long type.

n
     Immediate repeated type. Followed by the repeat count.

N
     Repeated type. Followed by the repeat count of the repeated type, followed by the type index of the repeated type. Due to a bug in g++ 2.7.2, this is only generated if index is 0. Superceded by n when squangling.

P
     Indicates a pointer type.  Followed by the type pointed to.

Q
     Used to mangle qualified names, which arise from nested classes. Also used for namespaces. In Java used to mangle package-qualified names, and inner classes.

r
     Encodes the GNU C++ long double type.

R
     Indicates a reference type.  Followed by the referenced type.

s
     Encodes the C++ and java short types.

S
     A modifier that indicates that the following integer type is signed. Only used with char.

     Also used as a modifier to indicate a static member function.

t
     Indicates a template instantiation.

T
     A back reference to a previously seen type.

U
     A modifier that indicates that the following integer type is unsigned. Also used to indicate that the following class or namespace name is encoded using Unicode-mangling.

u
     The restrict type qualifier.

v
     Encodes the C++ and Java void types.

V
     A modifier for a volatile type or method.

w
     Encodes the C++ wchar_t type, and the Java char types.

W
     Indicates the closing parenthesis of an expression.

x
     Encodes the GNU C++ long long type, and the Java long type.

X
     Encodes a template type parameter, when part of a function type.

Y
     Encodes a template constant parameter, when part of a function type.

Z
     Used for template type parameters.

The letters G, M, O, and p also seem to be used for obscure purposes ...

Virtual Tables
==============

In order to invoke virtual functions, GNU C++ uses virtual tables. Each virtual function gets an index, and the table entry points to the overridden function to call. Sometimes, and adjustment to the this pointer has to be made before calling a virtual function:

     struct A{
       int i;
       virtual void foo();
     };

     struct B{
       int j;
       virtual void bar();
     };

     struct C:A,B{
       virtual void bar();
     };

     void C::bar()
     {
       i++;
     }

     int main()
     {
       C *c = new C;
       B *b = c;
       c->bar();
     }

Here, casting from c to b adds an offset. When bar is called, this offset needs to be subtracted, so that C::bar can properly access i. One approach of achieving this is to use thunks, which are small half-functions put into the virtual table. The modify the first argument (the this pointer), and then jump into the real function.

The other (traditional) approach is to have an additional integer in the virtual table which is added to this. This is an additional overhead both at the function call, and in the size of virtual tables: In the case of single inheritance (or for the first base class), these integers will always be zero.

Virtual Base Classes with Virtual Tables
----------------------------------------

In case of virtual bases, the code is even more complicated. Constructors and destructors need to know whether they are "in charge" of the virtual bases, and an implicit integer __in_chrg for that purpose.

     struct A{
       int i;
       virtual void bar();
       void call_bar(){bar();}
     };

     struct B:virtual A{
       B();
       int j;
       virtual void bar();
     };

     B::B(){
       call_bar();
     }

     struct C{
       int k;
     };

     struct D:C,B{
       int l;
       virtual void bar();
     };

When constructing an instance of B, it will have the following layout: vbase pointer to A, j, A virtual table, i. On a 32-bit machine, downcasting from A* to B* would need to subtract 8, which would be the thunk executed when calling B::bar inside call_bar.

When constructing an instance of D, it will have a different layout: k, vbase pointer to A, j, l, A virtual table, i. So, when downcasting from A* to B* in a D object, the offset would be 12.

This means that during construction of the B base of a D object, a virtual table is needed which has a -12 thunk to B::bar. This is only needed during construction and destruction, as the full object will use a -16 thunk to D::bar.

In order to implement this, the compiler generates an implicit argument (in addition to __in_chrg): the virtual list argument __vlist. This is a list of virtual tables needed during construction and destruction. The virtual pointers are ordered in the way they are used during construction; the destructors will process the array in reverse order. The ordering is as follows:

  * If the class is in charge, the vlist starts with virtual table pointers for the virtual bases that have virtual bases themselves. Here, only polymorphic virtual bases (pvbases) are interesting: if a vbase has no virtual functions, it doesn't have a virtual table.

  * Next, the vlist has virtual tables for the initialization of the non-virtual bases. These bases are not in charge, so the layout is recursive, but ignores virtual bases during recursion.

  * Next, there is a number of virtual tables for each virtual base. These are sorted in the order in which virtual bases are constructed. Each virtual base may have more than one vfield, and therefore require more than one vtable. The order of vtables is the same as used when initializing vfields of non-virtual bases in a constructor.

The compiler emits a virtual table list in a variable mangled as __vl.classname.

Class with virtual bases, but without pvbases, only have the __in_chrg argument to their ctors and dtors: they don't have any vfields in the vbases to initialize.

A further problem arises with virtual destructors: A destructor typically has only the __in_chrg argument, which also indicates whether the destructor should call operator delete. A dtor of a class with pvbases has an additional argument. Unfortunately, a caller of a virtual dtor might not know whether to pass that argument or not. Therefore, the dtor processes the __vlist argument in an automatic variable, which is initialized from the class' vlist if the __in_chrg flag has a zero value in bit 2 (bit mask 4), or from the argument __vlist1 if bit 2 of the __in_chrg parameter is set to one.

Specification of non-thunked vtables
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In the traditional implementation of vtables, each slot contains three fields: The offset to be added to the this pointer before invoking a virtual function, an unused field that is always zero, and the pointer to the virtual function. The first two fields are typically 16 bits wide. The unused field is called `index'; it may be non-zero in pointer-to-member-functions, which use the same layout.

The virtual table then is an array of vtable slots. The first slot is always the virtual type info function, the other slots are in the order in which the virtual functions appear in the class declaration.

If a class has base classes, it may inherit other bases' vfields. Each class may have a primary vfield; the primary vfield of the derived class is the primary vfield of the left-most non-virtual base class. If a class inherits a primary vfield, any new virtual functions in the derived class are appended to the virtual table of the primary vfield. If there are new virtual functions in the derived class, and no primary vfield is inherited, a new vfield is introduced which becomes primary. The redefined virtual functions fill the vtable slots inherited from the base; new virtual functions are put into the primary vtable in the order of declaration. If no new virtual functions are introduced, no primary vfield is allocated.

In a base class that has pvbases, virtual tables are needed which are used only in the constructor (see example above). At run-time, the virtual tables of the base class are adjusted, to reflect the new offset of the pvbase. The compiler knows statically what offset the pvbase has for a complete object. At run-time, the offset of the pvbase can be extracted from the vbase pointer, which is set in the constructor of the complete object. These two offsets result in a delta, which is used to adjust the deltas in the vtable (the adjustment might be different for different vtable slots). To adjust the vtables, the compiler emits code that creates a vtable on the stack. This vtable is initialized with the vtable for the complete base type, and then adjusted.

In order to call a virtual function, the compiler gets the offset field from the vtable entry, and adds it to the this pointer. It then indirectly calls the virtual function pointer, passing the adjusted this pointer, and any arguments the virtual function may have.

To implement dynamic casting, the dynamic_cast function needs typeinfos for the complete type, and the pointer to the complete type. The typeinfo pointer is obtained by calling the virtual typeinfo function (which doesn't take a this parameter). The pointer to the complete object is obtained by adding the offset of the virtual typeinfo vtable slot, since this virtual function is always implemented in the complete object.

Specification of thunked vtables
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For vtable thunks, each slot only consists of a pointer to the virtual function, which might be a thunk function. The first slot in the vtable is an offset of the this pointer to the complete object, which is needed as a parameter to __dynamic_cast. The second slot is the virtual typeinfo function. All other slots are allocated with the same procedure as in the non-thunked case. Allocation of vfields also uses the same procedure as described above.

If the virtual function needs an adjusted this pointer, a thunk function is emitted. If supported by the target architecture, this is only a half-function. Such a thunk has no stack frame; it merely adjusts the first argument of the function, and then directly branches into the implementation of the virtual function. If the architecture does not support half-functions (i.e. if ASM_OUTPUT_MI_THUNK is not defined), the compiler emits a wrapper function, which copies all arguments, adjust the this pointer, and then calls the original function. Since objects of non-aggregate type are passed by invisible reference, this copies only POD arguments. The approach fails for virtual functions with a variable number of arguments.

In order to support the vtables needed in base constructors with pvbases, the compiler passes an implicit __vlist argument as described above, if the version 2 thunks are used. For version 1 thunks, the base class constructor will fill in the vtables for the complete base class, which will incorrectly adjust the this pointer, leading to a dynamic error.